Commitment is deciding on a course of action. Discipline is taking the actions you've committed to. Both are needed for success.
Some folks may have no trouble coming up with a plan, psyching themselves up for the challenge, etc. But, along comes the first thing they can't have because of their plan, and they fail at discipline.
It's best not to "beat oneself up" over slips, but at the same time, that shouldn't make slips "OK." We all have them, sure. We would rather we didn't.
So, decide! And then do! It's simple, but it's not easy.

Many have reached their goals this way!
Jay